A stem of a rotatable valve member, such as a butterfly valve, is weakened in certain areas such as, for example, the areas where pins secure the butterfly valve member to the stem. If an unusually high torque is placed on the stem, shear failure of the stem may occur and fluid pressure in the valve body under certain conditions may tends to urge or blow the sheared stem out of the bore resulting in possible injury to an operator of the valve. It is highly desirable that any device to prevent the blowout of a sheared stem be easily assembled within the bore of the valve body.